A. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to an in-plane switching mode liquid crystal display device.
B. Description of the Related Art
Recently, the thin film transistor liquid crystal display device (TFT LCD) has been used as a display device of many applications such as a portable television and a notebook computer, but such TFT LCD has a small viewing angle.
In order to solve this problem, twisted nematic LCDs having optical compensation plates and a multi-domain LCD, and so forth, have been introduced. In these LCDs, however, the color of the image is shifted because the contrast ratio depends on the viewing angle direction.
For a wide viewing angle, an in-plane switching mode LCD is disclosed, for example, in JAPAN DISPLAY 92 p547, Japanese Patent Unexamined Publication No. 7-36058, Japanese Patent Unexamined Publication No. 7-225388, and ASIA DISPLAY 95 P707.
FIG. 1a and FIG. 1b are respectively plane and sectional views showing the conventional in-plane switching mode liquid crystal display device (IPS mode LCD).
FIG. 1b is a sectional view taken along line A-A' of FIG. 1a. As shown in these figures, a gate bus line 1 and a data bus line 2 are formed on a first substrate 10, defining a pixel. Although only one pixel is drawn in the figures, a liquid crystal display device generally has a plurality of pixels. A common bus line 3 is aligned in the pixel, being parallel to gate bus line 1. A thin film transistor (TFT) is disposed at the cross of gate and data bus lines 1 and 2. As shown in FIG. 1b, the TFT comprises a gate electrode 5, a gate insulator 12, a semiconductor layer 15, an ohmic contact layer 16 and source/drain electrodes 6 and 7. In the pixel, a data electrode 8 and a common electrode 9 are formed parallel to data bus line 2. A portion of data electrode 8 which overlaps common bus line 3 is formed to obtain a storage capacitor which functions as maintaining a grey level voltage applied into data electrode 8. Common electrode 9 is connected to common bus line 3. Data electrode 8 is formed on gate insulator 12 and is connected to drain electrode 7. The TFT, data electrode 8 and gate insulator 12 are covered with a passivation layer 20. Thereon, a first alignment layer 23a is coated to impart an alignment direction.
On a second substrate 11, a black mask 28 is formed to prevent a leakage of light through the regions of the TFT and gate, data and common bus lines 1, 2, and 3. Thereon, a color filter layer 29 and a second alignment layer 23b are formed. Between first and second substrates 10 and 11, a liquid crystal layer 30 is formed.
When a voltage is applied to the conventional IPS mode LCD, an electric field parallel to substrates 10 and 11 is generated between data and common electrodes 8 and 9. Liquid crystal molecules in the pixel are rotated according to the electric field, controlling the amount of light passing through liquid crystal layer 30.
However, the conventional IPS mode LCD has the following problems. First, because the area for storage capacitor occupies quite a portion of the pixel region, and the data and common electrodes are made of opaque metals, the aperture ratio is lowered. Second, because the electric field applied to the LC layer is weakened by both gate insulator 12 and passivation layer 20 formed over two electrodes 8 and 9, the driving speed of the LC molecules is decreased, and consequently the driving voltage is increased. Third, because data bus line 2 should be apart from the pixel region to the extent of not generating the crosstalk problem, the pixel region is decreased, thereby lowering the aperture ratio.